The present invention relates to resin-fortified emulsion polymer compositions having improved mechanical and physical properties. Additionally, the present invention relates to methods of preparing these resin-fortified emulsion polymers.
High molecular weight emulsion polymers are gaining wide acceptance in the coatings industry. However, high molecular weight emulsion polymer technology has many drawbacks. Problems associated with high molecular weight emulsion polymer systems include poor wetting of metal substrates, cratering of resulting films, low gloss or mudcaking of resulting films, wetting and adherency problems, mechanical instability, freeze-thaw instability, shear thinning and poor physical properties of the resulting film. It is quite evident that a need exists to improve emulsion polymer systems.
U.S. Pat. No. 4,151,143, issued to Blank et al., discloses a surfactant-free polymer emulsion coating composition and a method for preparing the same. Blank et al. identify that the main problem with emulsion polymers employed for coatings is the presence of surfactants which are employed to stabilize the emulsions but which adversely affect the water and corrosion resistance of the resulting film as well as the adhesion of the coating especially to metal surfaces. The Blank et al. emulsion polymers are prepared in a two stage process. The process includes (1) a first stage wherein a conventional carboxyl group containing polymer is prepared by a conventional solution or bulk polymerization technique and thereafter water-dispersed or solubilized by partial or full neutralization thereof with an organic amine or base and application of high shear agitation and (2) a second stage wherein a mixture of polymerizable monomers and polymerization catalyst is added to the first stage emulsion at an elevated temperature to effect polymerization of the stage two monomers resulting in the formation of an emulsion coating composition. The coating composition is surfactant-free.
U.S. Pat. No. 4,179,417, issued to Sunada et al., discloses a composition for water based paints containing a water-soluble resin and a water-dispersible polymer. The water soluble resin contains 50-99.5 percent by weight of an alpha,beta-monoethylenically unsaturated acid alkyl ester or an alkenyl benzene, 0.5-20 percent by weight of an alpha,beta-monoethylenically unsaturated acid and 0-30 percent by weight of an hydroxyalkyl ester of an alpha,beta-monoethylenically unsaturated acid. These monomers are polymerized in the presence of an alkyd resin containing a polymerizable unsaturated group, epoxy esters containing a polymerizable group, drying oils, fatty acids of drying oils and diene polymers. The resulting polymers are water solubilized by the addition of ammonia or an amine. The water-dispersible polymer contains hydroxy and/or carboxyl functional groups and contains an alpha,beta-monoethylenically unsaturated acid monomer and/or a hydroxy alkyl ester of such a monomer and other ethylenically unsaturated monomers. The compositions disclosed in U.S. Pat. No. 4,179,417 are employed as water based paints and can optionally contain a crosslinking agent.
Canadian Pat. No. 814,528, issued June 3, 1969, discloses low molecular weight alkali soluble resin, resin cuts and methods of their preparation and purification. The resins are disclosed as being especially useful as emulsifiers, leveling agents and film-formers. The number average molecular weight of the resins range from 700-5,000 and the resins have acid numbers between 140-300. The resins are disclosed as emulsifiers in the preparation of emulsion polymers resulting in emulsion polymers which are stable and substantially free from coagulum. For this use, i.e., emulsifier in emulsion polymerization reactions, the resins must have a number average molecular weight between 1,000 and 2,000 and preferably between 1,000 and 1,500. Resins having a number average molcular weight greater than 2,000 result in unstable and coagulated emulsion polymers when used as the emulsifier in emulsion polymerization reactions.
Problems commonly associated wtih high molecular weight emulsion polymers include problems relating to mechanical stability, freeze-thaw stability, viscosity, pigment dispersity and wetting. In particular, the viscosity of high molecular weight emulsion polymers is shear dependent (non-Newtonian) and shear thinning (decrease in viscosity) occurs.
The resin-fortified emulsion polymers of the present invention provide extraordinary mechanical, physical and performance properties when compared to conventional emulsion polymers and represent a pioneering advance in emulsion polymerization technology. In particular, the resin-fortified emulsion polymers of the present invention possess (1) substantially Newtonian-like rheological properties (viscosity is shear independent), (2) excellent mechanical stability and freeze-thaw stability, (3) excellent pigment dispersity and (4) excellent wetting properties. Additionally, the present invention allows great flexibility in controlling the amount and type of resin added to the emulsion polymer compared to the Blank et al. emulsion polymers and the polymers disclosed in Canadian Pat. No. 814,528. The greater flexibility allows for the control of the properties of the final emulsion polymer product.